Acid process of fiber liberation



atente fiept. .1

GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR T0 BROWN COMPANY,OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE ACID PROCESS OF FIBERLIJBERA'I'ION No Drawing.

' This invention relates to the production from wood or other rawcellulosic materlals of a high grade chemical pulp characterized notonly by excellent physical characteristics,

5 is thencooked in a solution of'sul-phurous 5 including strength andtear resistance, but

also by easy bleachability, so that it may be converted into a whiteproduct suitable for high grade papers, with low bleach consumption andwithout .substantial injury to its ph sical or chemical characteristics.

'lhe so-called acid sulphite process of fiber liberation is employed forthe production of light-colored pulps which are bleachable to thewhiteness desired in high grade papers, without excessive degradation ofthe cellulose or injur to the fibers. In practising this process of herliberation, chipped wood or other raw cellulosic material is cookedunder 'conditions of high temperature and pressure in a liquorconsisting of a sulphurous acid solution of a sulphite of calcium,magnesium, sodium, or the like, containing about 4% to 7% free S0 whichis gradually released from the liquor during the cooking opera- 5 tion,while the temperature of cooking is increased.

in accordance with the process of the pres ent invention, fiberliberation is completed in a liquor of lower combined S0 content thanthat of the fresh liquor in which cooking is started, as I have foundthat such a process makes possible the production of a pulp of improvedcharacteristics, including higher strength and tear resistance andeasier bleachability. In carrying out my process, the raw cellulosicmaterial is initially cooked in the usual acid sulphite liquors or inacid sulphite liquors in which chemicals such as sodium sulphate sodiumbisulphate, borax, or the like, undergo progressive metatheticalreaction with sulphurous acid to produce sodium bisulphite. "Thisinitial cooking operation, however, is carried short of complete fiberliberation, whereupon the cellulosic material acid containing little, ifany, S0 in combined condition. The second cooking operation may becarried out at lower temperature than the initial cooking operation, forunder such lower temperature conditions a sulphur- Application filedoctoberae, 1929. seri itcueparer.

ntaining little, if any, comb ned S0 ismore reactive and more selectivein its actionupon hgneous constituents int cellulosic material than theusual acid su iquor. The'improved qualities of the ltiii'g' productoverthe usual sulphite pulp may be attributed to the greater hydrolysisof fiber occurring during the usual acid sulphite cook under theprogressively increasing temperatures employed to compensate for theremoval of free SO from the cooking liquor.

The following specific example of procedure will serve to illustrateconditions under which the process of the present invention may bepractised to produce a pulp havin markedly better characteristics thanthe usua sulphite pulp. Spruce or other wood chips are initially cookedwhile confined in a digester in an acid sulphite liquor containing asubstantial proportion of combined S0 The acid sulphite liquor may be ofthesodium base or the usual calcium base type, and contain 1% S0combined in the form of sulphite, and 5% S0 in free condition. I

When a sodium base is employed, it may be preferable to dissolvesuflicient sodium base chemical in the liquor to produce a much highercombined S0 concentration. therein than that of the less soluble calciumbase, for instance 3% to 4%, as a more rapid and "uniform penetration ofthe wood by combined S0 is ensured. Such higher combined S0concentration permits higher tem erature of cooking, as the combined S0in t e chips retards hydrolyzing action on the fibers by the free S0 Thetemperature of cooking in an acid sulphite liquor containing 1% combinedS0 and 5% free S0 shouldnot be higher than 300 F., a temperature of 275F. preferably being employed, but when an acid sulphite cooking liquorcontaining, say, 4% combined and 4% free S0 is employed, cooking may becarried out at 300 to 325 F, and these higher temperatures may bereached more quickly without danger of a non-uniform action and injuryto the fibers. After the chips and liquor have been brought to maximumcooking temperature, such temperature may be maintained for from two totill four hours, depending upon the composition of the particular liquoremployed and the maximum temperature of cooking. At the end of thisperiod, the chips are lighter in color than they were originally, butthey are still hard because they contain considerable non-fibrouscementitious material, and hence cannot be reduced to pulp unless theyare subjected to a severe grinding operation, such as practiced inmaking ground-wood pulp. The acid liquor is of light color and containsa. large proportion of unspent chemical, so that it may be separated, asby draining, from the cooked chips and again used for the cooking of rawwood, additional SO being dissolved therein, if desired, to compensatefor such loss of free S0 as took place during cooking and separation ofthe cooked chips. In order to minimize the loss of free SO from the acidcooking liquor, it may be separated from the chips under pressure bypumping it from the confined charge in the digester to another closeddigester in which the pulping of other raw wood is to be effected.

The cooked chips, while still hot and confined in the digester, may betreated with the second cooking liquor consisting of a solution ofpractically only sulphurous acid, but it is preferable to wash thembefore adding such cooking liquor to remove substantially all thecombined S0 associated therewith after cooking. The sulphurous acidsolution employed may contain 3% to 6% S0 it being preferable to employliquors of such free S0 asan excessive period of time is required forcompleting fiber liberation in more dilute solutions. When using aliquor containing, say, 5% free S0 cooking of the chips may be carriedout at a maximum temperature of 250 F. In order to avoid pressures above7 5 to 80 pounds, free S02 may be released from the liquor as cookingproceeds, the released gas being recovered in suitable systems, asordinarily. If cooking is effected at a temperature of 250 F. for aboutthree to five hours, the chips will be completely pulped, so that thedigester contents may then be blown into a pit, where the pulp may bedrained or washed free of cooking liquor. The free S0 arising from thepit during the blowing of the digester may be recovered for use in theproduction of fresh acid liquor, and the separated spent liquor may bemixed with spent liquor resulting from the initial cooking step, whenrecovery of their valuable constituents is practiced, as for instancewhen a sodium. base cooking liquorjs employed in the initial cookingstep.

A pulp produced as hereinbefore described is of high strength and tearresistance and has a high cooking stain value, indicating easybleachability. I present herein a table showing certain characteristicsof a pulp produced as hereinbefore described, and the usual commercialsulphite Product resulting from the Usual comprocess of the mercialsulpresent invenphite pulp tion Strength 150 125 Tear 180 150 Cookingstain 120 100 The cooking stain value of 120 indicates that the ulpresulting from the process herein descri ed will require only about 8%calcium hypochlorite bleach to be converted into a white product,whereas the sulphite pulp having a cooking stain of 100 will. consumeabout 15% of such bleach in acquiring the same degree of whiteness.

My process may be practised in a period of time not in excess of thecooking period of the usual acid sulphite process. Were it attempted insuch a period of time to produce by the usual acid sulphite process aproduct having the easy bleachability of the product resulting from myprocess, it would be necessary to complete fiber liberation at such hightemperatures as would cause serious degredation of cellulose and amarked lowering of the strength of the product. On the other hand, wereit attempted to produce pulp by cooking in sulphurous acid solutionsalone, it would be necessary to cook for exceedingly long periods oftime and at a temperature of about 220 F., or lower, owing to the severehydrolysis of cellulose and injury to the fibers taking place if fiberliberation is hastened by raising the temperature of the cooking liquor.Even when fiber liberation is effected at comparatively low temperatureby cooking in a liquor consisting of only sulphurous acid solution, theresulting pulp is so dark that it requires an excessive amount ofbleaching agent and undergoes such marked degradation during bleachingas to be of little value for papermaking or other purposes. Apparentlythe combined S0 content of the usual acid sulphite liquor reacts uponencrustations of the wood in a different way than a sulphurous acidsolution alone, this being borne out by the fact that after wood hasbeen cooked with a sulphurous acid solution alone, it cannot readily beconverted into fiber by the usual acid sulphite cook. Evidently asulphurous acid solution causes a resinification or polymerization ofcertain constituents present in the raw Wood to produce reactionproducts difiicultly soluble in the usual acid sulphite liquors, evenunder high temperature and pressure conditions. By first cooking thewood in an acid sulphite liquor containing combined S0 for instance theusualacid sulphite liquor, the production of such undesirable reactionproducts is evidently avoided, so that the fibers of the cooked chipsmay be readily liberated when fiber liberation is completed in asulphurous acid solution containing little, if any, combined SO toproduce a finished product havin the highly desirable characteristicsherein efore noted. 5 I claim:

1. A process which comprises cooking raw cellulosic material short offiber liberation in an acid sulphite liquor containing at least about1%combined S0 and then completing fiber liberation at lower temperaturein. a liquor containing substantially only free S0 2. A process whichcomprises cooking raw cellulosic material short of fiber liberation inrating the liquor from the cooked material,

peratures in a solution of substantially only sulphurous acid untilfiber liberation is complete. 2o 3. A process which comprises cookingraw cellulosic material short of fiber liberation in an acid sulphite'liquor, separating the liquor from the cooked material, washing thematerial, and cooking the material at lower temperature to completefiber liberation in a solution of substantially only sul phurous acid.

4. A process which comprises cooking raw cellulosic material short offiber liberation in an acid sulphite solution containing at least 1%combined S0 and at temperatures above 250 F., separating the liquor fromthe cooked material, and then cooking the material to complete fiberliberation in an as acid liquor containing less than 1% combined 0 andmore than 3% free S0 and at temperatures below 250 F.

5. A process of fiber liberation which con1- prises cooking chipped woodin an acid sulphite liquor suificiently short of fiber liberation sothat it contains considerable nonbrous cementitiousmaterial, and thencooking at lower temperature in a solution of substantially onlysulphurous acid to complete fiber liberation into a pulp.

In testimony whereof I have aifixed my signature.

GEORGE A. RICHTER.

